As a method of printing arbitrary characters on a medium serving as a printing target such as a paper, a passbook and the like, an impact dot printer of a wire dot system is widely known. In such an impact dot printer, the medium serving as the printing target is arranged between a print head and a platen. An ink ribbon impregnated with ink is arranged between the print head and the printing medium, and when printing, the ink impregnated in the ink ribbon is sequentially transferred to the medium. Since the ink is sequentially transferred to the medium, if a large number of characters are printed, the amount of the impregnated ink is decreased and thus the thickness of the printed characters becomes thinner and thinner. Thus, a reflection-type photo sensor is arranged on the print head, and it is also known that when the reflection-type photo sensor detects that the ink concentration of the printed characters becomes lower than a given value, the printing operations of the printer are stopped and the user is informed of that the ink ribbon is needed to be exchanged. However, the head used to carry out printing includes head pins that are used frequently and head pins that are not used much. The amount of the ink impregnated in the ink ribbon which is contacted with the head pins that are not used much is more than the amount of the ink impregnated in the ink ribbon which is contacted with the head pins that are used frequently. As a result, if the printing for confirming whether to exchange the ink ribbon is carried out at such a position where the ink ribbon is contacted with the head pins that are not used much, it may be determined that there is no need to exchange the ink ribbon, regardless of a state in which the ink ribbon should have been exchanged.